Recent studies suggest that fibroblasts from breast cancer patients differ in culture from fibroblasts of normal individuals and patients with benign breast disease. These differences can be seen in peritumor fibroblasts as well as fibroblasts obtained from sites far removed from the malignancy. Among these differences are the changes in phenotype often seen with fibroblast transformation, changes in motility, morphology, growth patterns, and serum dependency. These observations imply that cancer of the breast is not a focal disease but a systemic disorder, and that abnormal stromal-epithelial interactions are involved in the genesis and the maintenance of breast cancer. There are indications that these fibroblast abnormalities may even precede the appearance of frank malignancy and thus could provide important diagnostic markers for women at high risk for developing breast cancer. Because of the important diagnostic and mechanistic implications of finding such stromal abnormalities associated with breast cancer, we propose to expand on these observations by developing new assays which distinguish tumor-associated fibroblasts from those of normal individuals. We also propose to explore some basic biochemical mechanisms that might be responsible for the observed differences. A systematic examination of the components of the extracellular matrix will be made, including measurement of collagens, elastin, and proteoglycans, with particular emphasis on hyaluronic acid deposition and removal. Modulation of the deposition of these matrix components in cocultivation experiments will be examined, as well as the ability of various growth factors and tumor-derived transformation factors to emulate changes in fibroblast expression. These experiments will increase our understanding of the basic biology of breast cancer and the role of the host stroma in supporting the tumor process.